Messages in this thread | | | Date | Thu, 10 Aug 2023 09:49:14 +0800 | Subject | Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration() | From | Kefeng Wang <> |
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On 2023/8/10 6:44, Mike Kravetz wrote: > On 08/09/23 13:53, Mike Kravetz wrote: >> On 08/09/23 20:37, Kefeng Wang wrote: >>>> >>>> Cc Mike to help us clarify the expected behavior of hugetlb. >>>> >>>> Hi Mike, what is the expected behavior, if a user tries to use move_pages() >>>> to migrate a non head page of a hugetlb page? >>> >>> Could you give some advise, thanks >>> >> >> Sorry, I was away for a while. >> >> It seems unfortunate that move_pages says the passed user addresses >> should be aligned to page boundaries. However, IIUC this is not checked >> or enforced. Otherwise, passing a hugetlb page should return the same >> error. >> >> One thought would be that hugetlb mappings should behave the same >> non-hugetlb mappings. If passed the address of a hugetlb tail page, align >> the address to a hugetlb boundary and migrate the page. This changes the >> existing behavior. However, it would be hard to imagine anyone depending >> on this. >> >> After taking a closer look at the add_page_for_migration(), it seems to >> just ignore passed tail pages and do nothing for such passed addresses. >> Correct? Or, am I missing something? Perhaps that is behavior we want/ >> need to preserve? > > My mistake, status -EACCES is returned when passing a tail page of a > hugetlb page. >
As mentioned in previous mail, before e66f17ff7177 ("mm/hugetlb: take page table lock in follow_huge_pmd()") in v4.0, follow_page() will return NULL on tail page for Huagetlb page, so move_pages() will return -ENOENT errno, but after that commit, -EACCES is returned.
Meanwhile, the behavior of THP/HUGETLB is different, the whole THP will be migrated on a tail page, but HUGETLB will return -EACCES(after v4.0) or -ENOENT(before v4.0) on tail page.
> Back to the question of 'What is the expected behavior if a tail page is > passed?'. I do not think we have defined an expected behavior. If > anything is 'expected' I would say it is -EACCES as returned today. >
My question is,
Should we keep seem behavior between HUGETLB and THP, or only change the errno from -EACCES to -ENOENT/-EBUSY.
I would like to drop PageHead() check for Hugetlb to keep seem behavior, which will keep seem error code if isolate fail or success on head/tail page.
Thanks.
> BTW - hugetlb pages not migrated due to passing a tail page does not > seem to contribute to a 'Positive return value' indicating the number of > non-migrated pages.
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